In the intestinal tract of Crohn’s patients there is excessive synthesis with associated increased tissue levels of serotonin. In Crohn’s disease, high levels of serotonin
dominate synthesis, metabolism, and transport, leading to
dopamine and catecholamine levels that are low relative to
the balance needed to function properly with the serotonin
levels present.

The Article said:

The authors have documented a number of patients with Crohn’s disease who experienced similar remission of symptoms with this approach. This case was selected for this paper due to the severity of disease in the patient.

The Article said:

An imbalance of the serotonin–dopamine transport system has been linked to numerous diseases. It is proposed that much of the clinical constellation found with Crohn’s disease may be induced by a serotonin toxicity of the colon exacerbated by relatively low levels of dopamine resulting from defective OCTN transport.